The present invention relates to a process for preparing a porous film or sheet (both hereinafter referred to as "film") and more particularly to a process for preparing a porous film having a high flexibility, a high tearing strength and a high gas and moisture permeability and capable of being utilized in the fields of clothing, sanitary articles, medical supplies and filter medium.
It has heretofore been conducted to mix a polyolefin resin such as polyethylene with a filler, followed by melt extrusion, and then stretching the resulting film to obtain a porous film. However, a film obtained by using such mixture of a polyolefin resin and a filler is poor in stretchability, thus making it impossible to effect its stretching at a low stretch ratio, while when it is stretched at a high stretch ratio, the stretched film is low in tearing strength and its flexibility is poor. In view of these problems, various attempts for solution have been made. Many of those attempts involve the addition of a liquid compound in addition to the polyolefin resin and the filler. For example, in Japanese Patent Laid Open No. 47334/1982, liquid polybutadiene and liquid polybutene are used; in Japanese Patent Laid Open No. 203520/1982, liquid polyhydroxy-saturated hydrocarbons are used; in Japanese Patent Laid Open No. 149925/1983, liquid polyisoprene rubber is used; and in Japanese Patent Laid Open No. 140235/1984, epoxidated vegetable oils are used. In porous films obtained according to these methods, however, the liquid additives dissolve out when the films come into contact with solvents or chemicals. Even the polyolefin most flexible among those used in the above conventional methods is a linear low-density polyethylene having a density of 0.910 to 0.940 g/cm.sup.3, and thus the flexibility is still insufficient and a more flexible film has been desired.